Contrast media for gastrointestinal tract in roentogenologic examination include barium sulfate. In particular, barium sulfate is used in inspection of the gastrointestinal tract because it hardly transmits X-rays to provide a white image in a roentgenogram and, not being absorbed into a living body, is of very low toxicity (Encyclopedia Chimica, vol. 9, published by Kyoritsu Shuppan, Jan. 20, 1968, P.727-728). However, this method is disadvantageous in that a living body is exposed to a harmful X-ray and that barium sulfate causes side effects such as coprostasis since it is not absorbed into a living body.
MRI is a diagnostic technique comprising measuring the state of water present in a living body by utilizing nuclear magnetic resonance (NMR) of protons and imaging the distribution of water as a contrast.
The principle of information imaging consists in expression of the relaxation time T.sub.1 of the NMR phenomenon as a difference in image density. The shorter the T.sub.1, the whiter the image; and the longer the T.sub.1, the blacker the image. As the degree of restraint of water movement in a living body increases, T.sub.1 becomes shorter to provide a whiter MRI image as compared with free water. Further, T.sub.1 tends to be shortened in the presence of iron. In general, therefore, an iron compound is used as a contrast medium to be taken orally for MRI of the gastrointestinal tract, also taking safety to human bodies into consideration. For MRI of blood vessels, gadolinium compounds are employed as a contrast medium.
Where an iron compound, e.g., ammonium iron citrate, is used as a contrast medium for gastrointestinal tract MRI, it must be taken at a high dose amounting to several hundreds of milligrams. Such a large amount of an iron compound tastes of iron and will cause emotional stress in the living body and is therefore unsuitable for diagnosis of the gastrointestinal tract, such as the stomach. Other substances, if tried, have a fear of toxicity and, besides, they cannot be used in the preparation of food and drink for inspection particularly of the gastrointestinal tract because substances other than iron compounds are not admitted as a food additive.
Contrast media for diagnosis of gastrointestinal tract must be clearly distinguishable from living body tissues in the MRI image. Those for gastrointestinal tract inspection are particularly required to form a clear boundary with the wall of the gastrointestinal tract. In addition, because the contrast medium for gastrointestinal tract inspection is orally taken, toxicity of their components is of extreme importance. The components are preferably composed of nutrients.
Water in normal tissues generally approximates free water and is observed as a black MRI image. Accordingly, a contrast medium for MRI should be a substance which is to provide a whiter MRI image than free water's MRI image. To this effect, a contrast medium should comprise components which makes T.sub.1 shorter.
The inventors have noted the fact that T.sub.1 generally tends to become shorter in the presence of a paramagnetic substance. However, while a great number of paramagnetic substances are known, including transition metals, such as copper, manganese, cobalt, and chromium, various organic free radicals, oxygen, nitrogen dioxide, and so on, any of them involves the problem of toxicity and is not admitted as a food additive.